1. Field of the Invention
The present invention relates to a push switch used in input operation sections of various electronic apparatuses.
2. Background Art
Recently, electronic apparatuses have been reduced in size and thickness. Accordingly, push switches mounted on electronic apparatuses have been also reduced in size and thickness. Furthermore, push switches capable of providing an excellent click feeling at the time of operation have been demanded.
A conventional push switch is described with reference to FIGS. 8 to 12. FIG. 8 is a sectional view of a conventional push switch. FIG. 9 is an exploded perspective view of the conventional push switch. Case 1 has an opening on its upper part and is formed of an insulating resin. Center contact 2 is installed in the center of the inner bottom surface of the opening. Outer contacts 3 are installed in two places apart from center contact 2 with a predetermined distance. Terminal 2A is connected to center contact 2, and terminal 3A is connected to outer contact 3. Terminals 2A and 3A are integrated with case 1 formed of an insulating resin by insert molding. Terminals 2A and 3A are led out to the outside from the right and left side surfaces of case 1.
Movable contact 4 is formed of an elastic metal thin plate having an upward convex dome shape. The lower end of the outer periphery of movable contact 4 is placed on outer contact 3, and the lower surface of the center of movable contact 4 faces center contact 2 with a space therebetween. Movable contact 4 is accommodated in the opening of case 1.
Sheet 5 is an insulating film having flexibility. On the lower surface of sheet 5, an adhesive (not shown) is applied. Sheet 5 is fixed to the upper surface of case 1 with the adhesive, and covers the opening so as to protect center contact 2, outer contacts 3 and movable contact 4 from dust or the like.
Next, an operation of the conventional push switch is described. FIG. 10 is a sectional view of the conventional push switch mounted on an electronic apparatus. Terminals 2A and 3A of the push switch are packaged on the upper surface of wiring board 41 by soldering.
Operation button 42 includes pressing part 42A protruding downward and having a circular bottom surface, and operation part 42B protruding upward. Operation button 42 is formed of an insulating resin. Pressing part 42A is brought into contact with an upper surface of sheet 5. Operation part 42B protrudes upward from a hole provided at operation panel 43 of the electronic apparatus.
FIG. 11 is a sectional view showing a state in which movable contact 4 and center contact 2 comes into contact with each other in the conventional push switch. When a pressing force is applied to operation part 42B, the pressing force is applied to the center of movable contact 4 via sheet 5 that is brought into contact with pressing part 42A. When the pressing force is beyond a predetermined force, the center of the dome shape of movable contact 4 is elastically reversed in a downward-convex form with a click feeling and comes into contact with center contact 2. As a result, outer contacts 3 and center contact 2 become electrically conductive with each other via movable contact 4, and the switch is turned on.
Next, when the pressing force from the upper part is removed, the center of the dome shape of movable contact 4 elastically returns to its original upward-convex shape with a click feeling by elasticity of movable contact 4 itself. Accordingly, the center of movable contact 4 is separated from center contact 2, outer contact 3 and center contact 2 are insulated from each other, and the switch is turned off. A returning force of movable contact 4 allows operation button 42 to move upward via sheet 5, and the push switch returns to its original shape shown in FIG. 9.
FIG. 12 is a graph showing a relation between an operation distance and a pressing force in the conventional push switch. The axis of abscissas shows the operation distance and the axis of ordinates shows the pressing force. When a peak pressing force immediately before movable contact 4 is elastically reversed is denoted by P1 and a bottom pressing force immediately after movable contact 4 is elastically reversed is denoted by P2, the ratio of P1 to the difference between P1 and P2 is defined as a click rate. The click rate is represented by (P1−P2)×100/P1. When the click rate is 30% or more, a light click feeling is obtained. It is further preferable that the click rate is 40% or more and 60% or less.
When a conventional push switch is mounted on an electronic apparatus, a position of the push switch and a position of the electronic apparatus may be displaced from each other. When the center of pressing part 42A is displaced from the center of movable contact 4, the position that is displaced from the center of movable contact 4 is pressed, and thus the click feeling may be dull.
Furthermore, due to variation of the position displacement when the switch is mounted on the electronic apparatus, variation of switch feeling for each electronic apparatus may be increased.
Furthermore, it is demanded to increase an operation distance until the movable contact is elastically reversed so as to obtain a moderate stroke feeling as a feeling transmitted to the finger at the time of pressing.